Foil-form soldering metal and method for processing the same

ABSTRACT

The present invention provides a processing method capable of continuously working an Au—Sn soldering metal having a foil form in room temperature. The foil-form soldering metal containing from 10% by weight to 90% by weight of Au and balance comprising Sn is subjected to heat treatment for five minutes to ten hours at 200° C. to 270° C., and subsequently the foil-form soldering metal is slit. Thus, the heat treatment of the Au—Sn soldering metal before slitting enables continuous slitting of the Au—Sn foil-form soldering metal in room temperature and facilitates the production of a ribbon-form soldering metal.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for producing afoil-form soldering (filler) metal and, more particularly, to a methodfor processing a foil-form soldering metal for use in the process ofproducing a foil-form soldering metal having hard-to-work properties,for example, an Au—Sn foil-form soldering metal.

BACKGROUND ART

[0002] As a soldering metal for use in producing an optical device, suchas a die bond for a Ga, As chip, there is an Au-20 wt % Sn solderingmetal (referred to only as “An—Sn soldering metal” or only as “solderingmetal” below) comprising 80% by weight of Au (gold) and 20% by weight ofSn (tin). The Au—Sn soldering metal is processed to a foil form, whenusing it for example for the above-described die bond for a Ga, As chip.Generally, the soldering is automatically performed with a solderingmachine. A so-called ribbon-form Au—Sn soldering metal, which is a foilweb or elongated foil form, is needed for the continuous soldering.

[0003] The ribbon-form Au—Sn soldering metal has conventionally beenproduced for example in a following manner. A metal ingot having 80% byweight of Au and 20% by weight of Sn is first cast through melt-casting,and the resultant ingot is rolled out. A foil-form soldering metal (asoldering metal having a thickness of for example 20 μm to 100 μm)obtained via the rolling is slit to a desired width to obtain aribbon-form Au—Sn soldering metal.

[0004] However, the Au—Sn soldering metal as a workpiece, which isobtained through rolling an ingot, has properties of being brittle inroom temperature and easily cracked. Therefore, if the slitting isapplied to the soldering metal in room temperature, the soldering metaltends to be easily cracked at its edges during processing.

[0005] The crack occurs not only at the slitting but also at the rollingwhich is further performed when a thinner foil-form soldering metal isrequired, or at the blanking for producing a pattern-shape solderingmetal for use, for example, in sealing the seal part of IC packages.

[0006] If the crack occurs, the ribbon-form soldering metal tends to bebroken from the crack during or after processing. Therefore, it isdifficult to work into a ribbon-form soldering metal or a foil-formsoldering metal for sealing through the processing such as slitting,rolling or blanking in room temperature.

[0007] On the other hand, there is a processing method for hot workingthe foil-form soldering metal into a ribbon-form soldering metal or thelike, and crack is relatively hard to occur according to this method.Slitting is described below as an example. For example, when the Au—Snsoldering metal as a workpiece obtained through rolling an ingot has athickness of approximately 30 μm or more (generally 100 μm or less), aribbon-form soldering metal can be produced through applying theslitting having a width of approximately 0.5 mm. However, for hotworking such as hot slitting, a large scale apparatus is required, suchthat it is necessary to add facilities for heating an Au—Sn solderingmetal and working tools such as a slitting blade, a reduction roll and apunch for blanking, or it is necessary to put working devices in an oilbath. Further, the processability for handling the soldering metal isvery bad around the devices to which the heating facilities are added orwhich are in the oil bath. Therefore, the setting of the Au—Sn solderingmetal as a workpiece to the device is difficult. In particular, when thethickness is approximately 20 μm or less, the setting of the solderingmetal to the slitting device is difficult due to a low strength.Further, even if the setting is possible, the soldering metal tends tobe cracked or broken during processing, so that it is hard to work itinto a ribbon web or a pattern-shape soldering metal for sealing havinga narrow part. In addition, there is a disadvantage that the processingoperation becomes complicated such that regulation of the heatingtemperature is required for obtaining stable processing quality.

[0008] The present invention has been made in the above background, andit is an object of the present invention to provide a processing methodcapable of processing an Au—Sn soldering metal having a foil form inroom temperature.

DISCLOSURE OF THE INVENTION

[0009] In order to achieve the above-described object, the inventorshave studied the foil-form soldering metal as a workpiece obtainedthrough rolling. The results have revealed that the soldering metaltends to be easily cracked during the working such as slitting, rollingor blanking, because a hard and brittle intermetallic compound isproduced in the foil-form soldering metal containing 10% by weight to90% by weight of Au and the rest comprising Sn. In addition, it has beenfound that the Au—Sn soldering metal as a workpiece has a sectionstructure that looks elongated like a fiber (a white-looking ζ′ phaseand a black looking δ phase) (refer to FIG. 3), and that the presence ofsuch a structure is considered to be one of the causes for producingcracks during processing.

[0010] As a result of a further study based on these results, thepresent invention has been created by discovering that, for a foil-formsoldering metal as a workpiece obtained through hot working such as hotrolling, the foil-form soldering metal can continuously be processed inroom temperature without producing cracks or breaks by further applyingspecific heat treatment before processing.

[0011] Applying heat treatment before processing such as slitting to anAu—Sn foil-form soldering metal as a workpiece has never been studiedbefore, which is considered to be due to the following reasons. Thefirst reason is that a foil-form soldering metal as a workpiece is theone that is generally obtained through hot working such as hot rolling.Because it is reasonable to judge that the obtained soldering metal as aworkpiece is similar to the one which has already been subjected to heattreatment and there is no room for improving quality. The second reasonis that an Au—Sn foil-form soldering metal as a workpiece can be handledas a flexible material as long as it is not subjected to any type ofworking. Because it is reasonable to judge that the properties ofproducing cracks during processing is inherent in the Au—Sn foil-formsoldering metal, since an elongated foil-form soldering metal as aworkpiece can be handled as a flexible material when for example windingit to a reel or unwinding it from the reel, for example.

[0012] The present invention comprises a method for processing afoil-form soldering metal containing from 10% by weight to 90% by weightof Au and the rest comprising Sn, comprising: subjecting the foil-formsoldering metal to heat treatment for five minutes to ten hours at 200°C. to 270° C.; and then processing the foil-form soldering metal.

[0013] When the foil-form soldering metal as a workpiece is subjected tothe heat treatment having these conditions, the hardness of thesoldering metal is decreased (for example, Vickers hardness (Hv) isdecreased from approximately 180 to approximately 150) as well as thefiber-form structure in the soldering metal is eliminated to form(recrystallize to) a so-called island structure (refer to FIG. 4),thereby eliminating internal defects or distortions of the solderingmetal. Elimination of defects or distortions improves toughness of thesoldering metal to improve processability (stabilize mechanicalproperties). It is in what is called an elastic state (having body).

[0014] If it is possible to improve processability of the foil-formsoldering metal before processing in this manner, cracks duringprocessing in room temperature can be prevented. Thus, handlingproperties are improved, for example, the foil-form soldering metal caneasily be processed, as well as the yield of the processed product isimproved, facilitating the working, in particular, continuous working ofa soldering metal having a thickness of less than 30 μm which has beenhard to work. In addition, the breaks starting from cracks areeliminated, so that the handling of the foil-form soldering metal afterprocessing is improved. The processing method performed after the heattreatment includes, but not limited to, for example, slitting, rollingor blanking.

[0015] Various methods can be used for the heat treatment method of thefoil-form soldering metal as a workpiece. For example, a batch processis preferred when a separate sheet of the foil-form soldering metal isused, and when a web is used, the batch process or a continuous heattreatment process, in which the web is continuously fed into a furnace,is used as appropriate.

[0016] Further studies have been made on the processing method byfocusing attention on the fact that the processability of the foil-formsoldering metal as a workpiece is improved by applying such a heattreatment. The results have revealed that a thinner ribbon-formsoldering metal or a blanked soldering metal can be produced from afoil-form soldering metal after heat treatment, by further applyingrolling followed by applying slitting or blanking in room temperature. Aribbon-form soldering metal having a thickness of 20 μm or less, whichhas been hard to produce in the prior art, is easily produced by thismethod. For example, in the case of slitting, a foil-form solderingmetal as a workpiece having a thickness of 30 μm is subjected to heattreatment and then further rolled to form a thickness of 10 μm, which isthen slit. It is considered that the foil-form soldering metal furthersubjected to rolling after heat treatment can be slit in roomtemperature because the island structure produced by the heat treatmentremains.

[0017] Incidentally, when subjecting the foil-form soldering metal toheat treatment, the surface of the foil-form soldering metal may beoxidized (discolored). The oxidation of the surface of the foil-formsoldering metal may produce an unmelted portion or a poorly brazedportion during soldering. The results of a study on the problem haverevealed that it is preferred to perform the heat treatment of thefoil-form soldering metal in vacuum, in a H₂ (hydrogen gas) atmosphereor in an inert gas atmosphere such as Ar (argon gas) or N₂ (nitrogengas). Heat treatment in these atmospheres can securely prevent theoxidation of the surface of the foil-form soldering metal. The vacuumatmosphere is the lower the better, but conditions of 10⁻¹ Pa to 10 Paare actually used. The oxidation can be sufficiently prevented at 10 Paor below, and a vacuum of 10⁻¹ Pa has the same capability for preventingthe oxidation compared with the vacuum below 10⁻¹ Pa. On the other hand,the pressure is not limited in the case of a hydrogen gas atmosphere oran inert gas atmosphere, but the heat treatment of the foil-formsoldering metal is preferably performed in a space where the gas ispassed rather than in a space where the gas is in a stationary conditionto obtain a good quality product.

[0018] Furthermore, the processing method according to the presentinvention is more preferably used for the foil-form soldering metalcomprising from 29% by weight to 88% by weight of Au and the rest beingSn, because a higher effect of improvement can be obtained in thesoldering metal having such a component. In addition, it has been foundthat the holding temperature and the time for holding during the heattreatment before processing is more preferably in the range from 230° C.to 250° C. and from 30 minutes to 180 minutes, respectively. Because theheat treatment in these conditions can better insure the processabilityrequired for the workpiece for slitting, rolling or blanking,particularly in a shorter length of time of the heat treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a perspective view illustrating a state in which afoil-form soldering metal is wound on a reel;

[0020]FIG. 2 is a partial sectional view taken along line A-A of FIG. 1illustrating a state in which a foil-form soldering metal is being woundon a reel;

[0021]FIG. 3 is a photograph showing a cutting plane structure of afoil-form soldering metal before heat treatment; and

[0022]FIG. 4 is a photograph showing a cutting plane structure of afoil-form soldering metal after heat treatment.

BEST MODE FOR CARRYING OUT THE INVENTION

[0023] Preferred examples of the present invention will now be describedwith reference to the drawings.

[0024] An ingot of an Au-20 wt % Sn soldering metal was first producedvia melt-casting, which was processed into an elongated foil-formsoldering metal 10 by extrusion and rolling and was wound on a reel(made of SUS 304) 11 (refer to FIG. 1). The resultant elongatedfoil-form soldering metal 10 had a width of 20 mm, a thickness of 30 μmand a length of about 120 m. The outer diameter of a take-up spool 11 aof the reel 11 was 100 mm (refer to FIG. 2).

EXAMPLE 1

[0025] An example of slitting is described. The obtained foil-formsoldering metal web (a foil-form soldering metal as a workpiece) 10 wasfirst subjected to heat treatment with a furnace. After completed thewinding and before to be transferred into the furnace, a tape ofstainless steel (not shown) was wound on the outermost perimeter of thefoil-form soldering metal 10, and was fastened with a heat-resistantadhesive tape, thereby securing the foil-form soldering metal 10 so asnot to come loose from the reel 11.

[0026] As for the conditions of the heat treatment, the foil-formsoldering metal 10 was heated to 220° C. and then the temperature washeld for 120 minutes. Subsequently, it was allowed to cool in thefurnace. In addition, H₂ gas (hydrogen gas) was passed around the reel11 on which the elongated foil-form soldering metal 10 is, wound duringthe heat treatment. Note that the obtained elongated foil-form solderingmetal 10 may be rewound on another reel 11 for the purpose of forexample adjusting the length of the foil-form soldering metal to besubjected to heat treatment.

[0027] After the heat treatment, the foil-form soldering metal 10 wasunwound from the reel 11, and distortions such as waveform werecorrected via an ironing step. After that, a plurality of ribbon-formfoil-form soldering metal having a width of 0.3 mm were obtained throughslitting with a multi-blade slitter. A hot plate heated to 240° C. wasused in the ironing step.

EXAMPLES 2 TO 5 AND COMPARATIVE EXAMPLE 1

[0028] Ribbon-form foil-form soldering metals were obtained throughperforming heat treatment by use of the heat-treatment conditionsdifferent from Example 1. The conditions of the heat treatment for eachof the examples are shown in Table 1. No heat treatment was performed inComparative Example 1. All conditions other than the heat-treatmentconditions were the same as those in Example 1, so that the descriptionis omitted. TABLE 1 Heat treatment Evaluation of slitting conditionsBreak of Evaluation of Examples/ Tem- ribbon cross-section Comparativeperature Time during (evaluation of Overall Example (° C.) (min)processing crack) evaluation Example 1 200 180 No Good Good Example 2220 120 No Good Good Example 3 240  60 No Excellent Excellent Example 4260  20 No Good Good Example 5 270  5 No Good Good Comparative — — Yes —Poor Example 1

[0029] In Comparative Example 1 in which no heat treatment wasperformed, the ribbon was broken due to frequent occurrence of cracksduring slitting, and it was impossible to obtain a ribbon having alength exceeding one meter. On the other hand, in the case of any of theexamples, it was possible to perform good slitting with no break of theobtained ribbon, during slitting of the foil-form soldering metal havingd length of 120 m. These results have revealed that good heat treatmentcan be performed when the temperature of the heat treatment is in therange of 200° C. and 270° C. A longer length of time of the heattreatment is more preferable for securing the heat treatment in thistemperature range, but no difference was found in the resultant effecteven if the heat treatment is performed longer than 10 hours.Specifically, it has been found that the heat treatment for 180 minutesis sufficient. It has also been found that the higher the heat-treatmenttemperature, the heat treatment can be performed in shorter time.Specifically, it was found that, in the case of 270° C., the heattreatment time of five minutes provides a necessary and sufficienteffect for obtaining an elongated ribbon-form soldering metal.Furthermore, these results have revealed that also in the blanking forcutting the foil-form soldering metal into a specific shape, the heattreatment of the above-described conditions before processing improvesprocessability to improve handling properties and the yield.

[0030] The cutting plane of the ribbon-form soldering metal obtainedfrom each of the above-described examples was observed. The evaluationof the cross-section was good to excellent with almost no break beingfound in any of the foil-form soldering metals, among them the oneobtained in Example 3 having extremely few cracks. These results haverevealed that the conditions of the heat treatment before processingmost preferably have a holding temperature of from 230° C. to 250° C.and a holding time of from 30 minutes to 90 minutes.

EXAMPLE 6

[0031] An example of rolling is described. Heat treatment conditions andan ironing step are the same as described for the slitting method inExample 1. In this example, the foil-form soldering metal after theironing was fed into a rolling mill and hot rolled to obtain a foil-formsoldering metal having a thickness of 10 μm, a width of 20 mm and alength of about 360 m. That is to say, the reduction ratio (=(thicknessbefore rolling−thickness after rolling)/thickness before rolling×100)was about 67%. The obtained elongated foil-form soldering metal waswound on another reel.

EXAMPLES 7 TO 10 AND COMPARATIVE EXAMPLE 2

[0032] Heat treatment was performed by use of the heat-treatmentconditions different from Example 6, followed by rolling. The conditionsof the heat treatment for each of the examples are shown in Table 2. Noheat treatment was performed in Comparative Example 2. All conditionsother than the heat-treatment conditions were the same as those inExample 6. TABLE 2 Heat treatment conditions Examples/ Tem- Evaluationof rolling Comparative perature Time Rolling Evaluation of OverallExample (° C.) (min) yield (%) crack evaluation Example 6 200 180 100Good Good Example 7 220 120 100 Excellent Excellent Example 8 240  60100 Excellent Excellent Example 9 260  20 100 Good Good Example 10 270 5 100 Good Good Comparative — — 10 Poor Poor Example 2

[0033] In the case of Comparative Example 2, a large number of fissuresoccurred in the foil-form soldering metal obtained through rolling, sothat only about 40% (about 140 m) of them were usable for a foil-formsoldering metal. On the other hand, in the case of the examples, goodrolling was performed in any of the examples without large fissures onthe surface of the foil-form soldering metal. The surface of thefoil-form soldering metal obtained in each example was observed. Theevaluation of the surface was good to excellent with almost no fissures(cracks) being found in any of the foil-form soldering metals,particularly those obtained in Example 7 and 8 (holding temperature ofthe heat treatment was from 210° C. to 250° C., and holding time wasfrom 30 minutes to 150 minutes) being the best with extremely fewfissures. These results have revealed that similar to the case ofslitting, also in the case of rolling, the heat treatment of theabove-described conditions before processing improves processability,provides a soldering metal having excellent quality without fissures(cracks) and improves the yield.

EXAMPLE 11

[0034] One example of slitting is described. Specifically, a method forslitting a foil-form soldering metal obtained through rolling after heattreatment is described. Rolling conditions, including heat-treatmentconditions, were the same as Example 8, and slitting conditions were thesame as Example 1. That is to say, according to the present example, thefoil-form soldering metal obtained in Example 8 was slit with themulti-blade slitter used in Example 1. TABLE 3 Evaluation of slittingThickness Evaluation of Rolling of cross-section after heat soldering(evaluation of Overall Example treatment metal (μm) Sitting yield (%)crack) evaluation Example 11 Yes 10 100 Good Good

[0035] In Example 11, no break of the obtained ribbon occurred duringslitting of the foil-form soldering metal of 360 m, so that the slittingyield was 100%. This result has revealed that good slitting can beapplied to the foil-form soldering metal having a reduction ratio ofabout 67%.

EXAMPLE 12

[0036] One example of blanking is described. Specifically, the foil-formsoldering metal obtained through rolling after heat treatment wasblanked. Conditions of the heat treatment before rolling, conditions ofironing step and conditions of the rolling were the same as Example 8.The foil-form soldering metal after the ironing step was fed to ablanking machine to obtain a ring-form soldering metal having an innerdiameter of 0.3 mm, an outer diameter of 1 mm and a thickness of 10 μm.

COMPARATIVE EXAMPLE 3

[0037] Blanking without heat treatment is described. This example wasdifferent from Example 12 in having no heat treatment and no ironingstep, but was the same as Example 12 in rolling conditions. TABLE 4Evaluation of blanking Heat treatment Evaluation of Examples/ conditionsBlanking shear plane Comparative Temp- Rolling yield (evaluation ofOverall Example perature Time (min) yield (%) (%) crack) evaluationExample 12 240 60 100 100 Good Good Comparative — — 10 10 Poor PoorExample 3

[0038] In Comparative Example 3 which was not subjected to heattreatment, cracks frequently occurred on the shear plane of thesoldering metal due to blanking, and the ring-form soldering metal wasobtained only in about 10%. It has been found that the rolling yieldbefore blanking is also low in the case of no heat treatment, resultingin extremely low productivity. Compared with this result, the yield forExample 12 was 100%, and it was able to apply good blanking. The shearplanes of the soldering metals obtained in Example 12 and ComparativeExample 3 were observed. The foil-form soldering metal of the exampleexhibited good results with almost no cracks, but the soldering filermetal foil obtained in Comparative Example 3 exhibited a large number ofcracks. These results have revealed that also in the case of blanking,similar to slitting, the heat treatment of the above-describedconditions before processing improves processability, provides asoldering metal having excellent quality without cracks and improves theyield.

[0039] Advantages of the Invention

[0040] As apparent from the above description, in accordance with theprocessing method according to the present invention, an Au—Sn foil-formsoldering metal can be processed without producing defects such ascracks in room temperature.

1. A method for processing a foil-form soldering metal containing from10% by weight to 90% by weight of Au and balance comprising Sn,comprising: subjecting the foil-form soldering metal to heat treatmentfor five minutes to ten hours at 200° C. to 270° C.; and subsequentlyprocessing the foil-form soldering metal.
 2. The method for processing afoil-form soldering metal according to claim 1, wherein the processingafter heat treatment is slitting, rolling or blanking.
 3. The method forprocessing a foil-form soldering metal according to claim 2, comprisingthe step of rolling after heat treatment before slitting or blanking. 4.The method for processing a foil-form soldering metal according to anyone of claims 1 to 3, wherein the heat treatment of the foil-formsoldering metal is performed in vacuum, in a hydrogen gas atmosphere orin an inert gas atmosphere.